1. Field of the Invention
The present invention relates to an image forming optical system comprising a single aspherical plano-convex collimator lens for use as a pickup optical system for an optical disk or as an optical system for use in a laser beam printer.
2. Description of the Prior Art
For a lens system used for reproducing signals recorded on optical disks, a large aperture with a numerical aperture NA of approximately 0.45 to 0.5 is required when the light source is a semiconductor laser. It is also required that axial aberrations should not exceed the diffraction limit. Moreover, in order to cope with automatic focusing or tracking, it is required that the number of lens elements should be as small as possible.
Lens systems fulfilling these requirements have already been proposed in Japanese Laid-open Patent Applications Nos. S60-181714, S61-88213, S61-132915 and S63-192006. These lens systems are all excellent lens systems each of which has achieved its object with a single lens by use of the recent progress in aspherical surface technology.
Regarding single aspherical collimator lenses made by a glass molding method, there is a constraint condition for each of the lens material (on which refractive index depends), the axial thickness and the curvature. These conditions are due to the manufacture method.
A lens system disclosed by the above-mentioned Japanese Laid-open Patent Application No. S61-88213 fulfills the following conditions (1) to (3): EQU 1.2&lt;q&lt;1.5 (1) EQU 1.68&lt;n (2) EQU 0&lt;dg (3)
where q is defined by ##EQU2## f represents a focal length of the single lens, d represents an axial thickness thereof, n represents a refractive index thereof, NA represents a numerical aperture thereof, and dg represent a thickness of a plane parallel plate.
In this prior art, by setting (d+dg)/f in accordance with the condition (1), coma is restrained, and the unfulfilled amount of sine condition is reduced to zero. Specifically, coma in the vicinity of the optical axis is removed by preventing the maximum image height of the single plano-convex lens from decreasing, and the unfulfilled amount of sine condition in the pupil is reduced to zero.
However, in the aspherical plano-convex lens fulfilling the above conditions, a ratio d/D of the thickness d to the effective aperture D is inevitably large. This is because d+dg is inevitably large in a lens having a large focal length since (d+dg)/f should be within a predetermined range in order to bring coma into an excellent condition. For example, in an embodiment which is a collimator lens where dg=0.25, the focal length f is long (7 to 13) and the axial thickness d is extremely large (4.2 to 7.8).
When the lens is processed by press molding, since the cut off amount at the process where the lenses are cut off for determining the center is large, the cost of lens manufacture is high. When decentering occurs in the lenses which have been cut off for determining the center, the angle of view increases because of the decentering. In a lens where the ratio d/D of the axial thickness d to the effective aperture D is large, the amount of lateral aberrations due to the increase in angle of view is more apt to be large than in a lens where the ratio d/D is small. That is, the amount of aberrations increases highly responsively to a change in angle of view. Since a lens where the ratio d/D is large must be designed so that lateral aberrations are restrained in a large view angel range for this reason, the degree of freedom in lens designing such as the selection of a lens material is limited.
If the lens where the ratio d/D is large is designed so that lateral aberrations are restrained only in a small view angle range, the assembly will be extremely difficult since it is necessary to attach the lens to a lens barrel with a high positioning accuracy so that the decentering amount from the optical axis is small.
Generally, a single lens is positioned with either of the effective surfaces as a reference surface. This is because edge surfaces are not highly precisely processed. For this reason, even though the finished lens has an ideal shape, the surface reverse to the positioning surface decenters due to the work accuracy of a supporting portion of the lens barrel. Although the element of the inclination is the same whether the ratio d/D is large or small, the parallel decentering amount is largely different.